1. Field of the Invention
The invention relates to an injection molding assembly for foam products, more particularly to a fully automatic injection molding assembly which can control automatically the amount of foamable material to be injected into a mold unit and which can control automatically the opening of the mold unit.
2. Description of the Related Art
Presently, conventional chemical foam forming techniques require a direct compression molding step or the injection of foamable material into a compression mold to form foam products. These foam forming techniques cannot be accomplished at a fast pace because they require the presence of human operators, thereby preventing an increase in the production capacity to prevent correspondingly a decrease in the manufacturing costs. Furthermore, a large amount of scrap is produced when these techniques are performed.
FIG. 1 is a schematic view of a conventional injection molding assembly. In this figure, raw foamable material is fed into an injection cylinder 2 of an injection molding machine by means of a material hopper 22. A reciprocating screw 1 extends rotatably into the injection cylinder 2 and has an outer surface which is provided with heating elements (not shown). The reciprocating screw 1 is responsible for heating and homogenizing the foamable material in the injection cylinder 2 and for controlling the flow of the foamable material in the latter. When an injection chamber 21 located at a front end of the injection cylinder 2 already contains a predetermined amount of molten foamable material, a hydraulic cylinder 3 forces the reciprocating screw 1 toward a nozzle 20 of the injection cylinder 2, thereby forcing the molten foamable material in the injection chamber 21 into a mold cavity 231 of a mold unit 23. In order to prevent reverse flow of molten foamable material in the injection chamber 21, a nonreturn flow valve assembly 12 is installed on a front end of the reciprocating screw 1. When the reciprocating screw 1 rotates, the molten foamable material flows into the injection chamber 21 via a clearance 13 defined cooperatively by the valve assembly 12 and the internal wall surface of the injection cylinder 2.
Use of the conventional injection molding assembly requires precise control of the injecting pressure, the amount of molten foamable material to be injected, and the room and mold temperatures in order to obtain high quality foam products. Defective foam products are produced if any of these factors cannot be controlled.
In the conventional injection molding assembly, the values of the injecting pressure, the amount of molten foamable material to be injected and the mold temperature are determined in accordance with the operator's experience or by conducting repeated experimentation until the best quality product is produced. Thus, setting of these values cannot be accomplished quickly. Furthermore, after the injection molding assembly has been in use for a long time, changes in the operating conditions often results in changes in the heat dissipating and heat collecting properties of the mold unit, thereby making precise control of the mold temperature difficult to accomplish. This can affect the stability of the foam products. In addition, the evenness and smoothness of the foam products are affected if the amount of molten foamable material that is injected into the mold cavity cannot be controlled precisely.